First principles and molecular dynamics simulation investigation of mechanical properties of the PTFE/graphene composites

In this study, the dynamic molecular structure changes in polytetrafluoroethylene (PTFE) and PTFE/graphene composites under compression were investigated. The critical formation and fracture lengths of the C-C bond in PTFE, graphene, and between PTFE-graphene were calculated from first principles. Then, molecular models of these materials were constructed through molecular dynamics simulations. Finally, the critical lengths of the C-C bonds were embedded into the molecular models to study the effect of the molecular structure change during compression on the mechanical properties of PTFE and its composite. Further, molecular models with different defect ratios (1-30%) were constructed to simulate fracture damage stages upon increasing the PTFE and PTFE/ graphene composite service time. The results show that compression can induce fracture and formation of chemical bonds. The number of bonds broken is larger than the number of bonds formed; thus, the fracture damage increases. The molecular model stress decreases after bond formation, and the compression resistance of the composites decrease in the plastic deformation stage. With increasing defect ratio, the molecular model stress and load-carrying capacity decrease. The compression-induced bond formation decreases the compressive resistance to some extent and increases the tensile strength considerable.